Optical position encoders are used to accurately measure mechanical displacements between components of any apparatus undergoing movement, vibration, etc. The optical position encoder typically includes a movable code strip, with an optical track comprised of alternating transparent and opaque bars, that passes through and moves relative to an encoder sensor comprised of an illumination source, typically a light emitting diode (LED) and sensor array composed of a plurality of photosensitive elements usually photodiodes.
As the code strip moves, it interrupts the light from the illumination source causing an alternating series of light and shadow to pass across the sensor array producing electric signals that vary in amplitude. Circuitry in the encoder sensor amplifies the signals to produce two output signals in quadrature, phase-A and phase-B which is phase shifted from phase-A by 90 electrical degrees. As the code strip moves, this quadrature signal varies in frequency proportional to the speed of motion, and the phase relationship between phase-A and phase-B indicates the direction of motion.
Detection of the relative motion of components allows systems to provide adaptations and adjustments based upon the displacements. In some applications mechanisms that move through small displacements use an array of strain gauges bonded to flexible movable components. When correctly calibrated the detected variations in strain as those components flex provides an indirect measure of the motion. Strain gauges tend to be small and delicate components that need to be carefully and accurately applied to the surface being measured, attached using adhesive, protected with an additional coating, and cannot be easily replaced if damaged. Using an optical position encoder allows direct measurement of these small displacements and eliminates the complex assembly procedures involved when using strain gauge arrays. However, like the strain gauges, the components of an optical encoder also need to be accurately aligned to operate properly.
Typically, the mounting for the encoder involves mounting the encoder sensor on one base to one component or portion of an apparatus. The code strip generally has a separate mounting or base and needs to be accurately positioned relative to the encoder sensor. This results in two separate assemblies with two separate fastener systems. For field replacement, a service technician would have to carry two separate units and diagnose and replace them independently. The resulting complexity of both manufacture and service is undesirable.